1. Field of the Invention
The present invention relates to a method of assembling a motor vehicle body by welding together motor vehicle body constituent parts such as a floor panel, right and left side panels, a roof panel or the like at a plurality of tack-welding portions, and also relates to an apparatus to be used in carrying out the above-described method.
2. Description of Related Art
In a motor vehicle body assembly line, welding is carried out first at a plurality of tack-welding portions required to hold a plurality of motor vehicle body constituent parts together in proper orientation, and then reinforcing welding is carried out to secure the strength of the motor vehicle body.
In welding the tack-welding portions, the plurality of motor vehicle body constituent parts are fixed or set in position relative to each other by means of positioning members provided on welding jigs which are disposed on both sides of an assembly station and then, in this condition, welding of the tack-welding portions is carried out by a plurality of welding guns mounted on the welding jigs.
It has hitherto been so arranged that the welding jigs are provided with welding transformers to be connected to welding guns mounted on the welding jigs and that electric power is directly supplied from these welding transformers to the welding guns. However, this arrangement has a disadvantage in that the welding jigs become large in size. As a solution to this disadvantage, the following is known. Namely, the welding guns to be mounted on the welding jigs are arranged by guns of indirect power supply type, and feeder terminals (or electric power supply terminals) which are connected via feeder cables (or electric power supply cables) to the welding guns are fixed to terminal racks which are disposed outside the welding jigs. Each of these feeder terminals is pinched by a portable type welding gun or the like to thereby supply electric power to each of the welding guns mounted on the welding jigs (see Japanese Published Unexamined Utility Model Registration Application No. 10984/1984, Japanese Published Unexamined Patent Application No. 63577/1988 and Japanese Published Examined Patent Application No. 13115/1991).
The welding jigs of a particular design are prepared for each kind of motor vehicle. It follows that they have to be changed for another design when the kind of motor vehicle is changed.
It has hitherto been so arranged that all of the tack-welding portions are welded by the welding guns mounted on the welding jigs. The number of the welding guns to be mounted on the welding jigs therefore becomes large and, as a consequence, the cost of the welding jigs becomes high even if they are formed in the indirect power supply type. It is also necessary to prepare in advance a plurality of welding jigs in preparation for the change in the kind of the motor vehicle, resulting in an extremely high cost.
In this case, the following arrangement may be considered. Namely, only the positioning members are mounted on the welding jigs, and the welding of the tack-welding portions are made by welding robots. In this solution, however, the plurality of tack-welding portions are sequentially pinched together to perform welding. Consequently, there will occur a positional deviation or displacement in those tack-welding portions which lie behind in the welding procedure, due to distortions which may occur when those tack-welding portions which lie ahead in the welding procedure (or in the order of welding) are tack-welded.
In view of the above-described disadvantages, the present invention has an object of assembling a motor vehicle body with a higher accuracy while reducing the equipment cost.